Liquid-liquid separators are used in a variety of applications to separate two liquid phases having different densities. Liquid-liquid separators may be used to separate the solvent and polymer in the Solution Polyolefin process. For the stability of a solution, it is very important to control the interface layer (the rag layer) between the polymer solution and pure solvent. The location of this layer determines the degree of separation between the two liquids. Unfortunately, the separator does not contain any side glass to view the interface and a sonic probe was used to detect the polymer and solvent interface in the carbon steel vessel.